1. Field of the Invention
The invention relates generally to a device configuration and method of protecting the device. More particularly, this invention relates to an improved circuit configuration and method of protecting active devices operated at a high voltage from damages due to over current caused by an electrical short by using a conductive material with a positive temperature coefficient.
2. Description of the Relevant Art
Under high power conditions, the power devices often fail in a “short circuit” condition and that can result in damages to other components in the system where the power devices are implemented. Therefore, it is preferable that a power device fails in an open circuit state and even more preferable that a failure can be avoided entirely. Prevention of failure can be achieved by integrating a protective circuit as part of the device or outside of the device. One particular protection implemented in a semiconductor device is to place a fusible link such as a metal fuse or a polysilicon fuse shown in FIG. 1A in the interconnect pattern before the bond pads. However, as shown in FIG. 1B such configuration has a limitation that for very high current application, multiple fuses and pads would be required thus causing an increase in die size and that leads to unfavorable cost impacts in manufacturing and in operation. Another disadvantage associates with multiple fuses and pads is that in case not all fuses blow open during an over-current condition, the active region of the device where the fuse did not blow open would be damaged. Furthermore, the addition of the protective circuit such as metal fuse, poly-fuse, or other “fail-open” protective circuits, often adversely affects the device or system performance. The fuse protection also has a disadvantage that such protection is not re-settable, once the fuse is broken, the fuse connection is broken even the over current condition is then eliminated.
In order to overcome such limitations, materials of a positive temperature coefficient (PTC) are implemented as re-settable over current protection device. Various PTC materials are known such as a polymer PTC (PPTC), which is commercially available in the market as “PolyFuse”, “Polyswitch” and “Multiswitch”. The products may be provided as a slice of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. When the plastic heats up, it expands, forcing the carbon grains apart, and causing the resistance of the device to rise rapidly. Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response and is used for switching, not for proportional temperature measurement. The PTC application is disclosed in U.S. Pat. No. 4,238,812 and various data sheets that provide the PTC materials as commercial products. Effectiveness of PTC protections has been demonstrated with an increase in resistance of up to five orders of magnitude when the temperature is increased.
Even though the application of materials of a positive temperature coefficient (PTC) for over-current protection of electronic devices is well known, there are still technical limitations and difficulties when the PTC materials are actually implemented. The PTC protection circuit is usually achieved by connecting the protection device as in FIG. 1C comprising PTC a material that has a resistance increases with temperature to a load. However, in order to achieve the protection, self-heating is usually required to increase the temperature and that requires an I2R drop within the device with a special mounting to avoid the heat sink that could reduce the effectiveness of device protection. In the meantime, higher resistance can induce more self-heating to achieve better protection. However, that negatively impacts the performance of the power systems. Alternatively, external heating may be used to heat the PTC materials by adding more circuit components but the added circuit components cause the current limiting protection to occupy large volume.
Another difficulty encountered in the protection of device applying the PTC materials is in the device operated at higher voltage. For the sake of reducing power consumption and increase operational efficiency, it is generally desirable to reduce the resistance of the protection circuits. However, as the resistance of the protection circuit is reduced, the voltage rating of the protection also tends to reduce. For some application with devices necessary to operate at a high voltage rating, the reduction of voltage rating of the protection circuit implemented with the materials with a positive temperature coefficient (PTC) becomes a major drawback and limitation. In addition, as the PTC protection circuit turns off, it takes on the applied (drain-source) voltage. Therefore, the voltage rating of the PTC device needs to equal or exceed the applied voltage. For higher voltage applications, this requires a PTC device to have correspondingly high voltage rating, which increases the resistance of the PTC device, even when during normal operation.
Specifically, a patent application Ser. No. 11/657,862 filed on Jan. 25, 2007 discloses a new device configuration, the disclosures of application Ser. No. 11/657,862 is hereby incorporated by reference in this Application. The application Ser. No. 11/657,862 discloses device configuration that has a PTC protection layer connected in series with the drain electrode of the active device. The device configuration however has a drawback that the voltage rating of the PTC structure has to equal or exceed the applied voltages thus limiting the scopes of applications of this type of protection circuits.
Therefore, a need still exists in the fields of circuit design and device manufactures for providing a new and improved configuration and manufacturing method to resolve the above-discussed difficulties. Specifically, a need still exists to provide new and improved configuration to implement the PTC protection such that the above discussed limitations and difficulties can be resolved.